Been volatile all her life ... So what boundaries have you, as a parent, already established? Because this doesn't sound like your DD is suddenly exhibiting behaviours that you find difficult, instead it seems that she's behaving like she always has and, now she's 20, you'd like rid and want some reassurance that she'll 'get help' from others.
My DD has autism and ADHD. She's a bit older than yours. No, there isn't any help. We, as parents, were (and still are) the help. She did move out at one point (of her own free will) and later came back home severely underweight and mentally in shreds. However, the 6+ years in weekly face to face therapy (that we paid for) mean she's now able to regulate her emotions, hold down a full-time professional (extremely well paid) job and enjoy a stable relationship with her partner of several years.
One of the keys to successfully parenting my DD was recognising the various comorbidities, e.g. the links between autism/ADHD and PMDD. This helped us to help her seek medical interventions. Same with OCD. In other words, once we understood what was going on, we were able to get the right treatment, which in turn improved everyone's lives enormously. Obviously, this was a process and took time throughout adolescence.
But to answer your question, how long are you supposed to put up with it? I work on the basis that I'll always be the parent, no matter how old my kids are, I'm their mum - and they're all around 30ish. This is probably because one of my kids is severely disabled and so I never imagined a life where I wouldn't be caring.